The biosynthesis of pro-opiocortn has been further studied with an emphasis on post-translational processing mechanisms. A pro-opiocortin converting enzyme activity has been detected in secretory vesicles from anterior, intermediate, and posterior pituitary lobes and partially characterized. The role of glycosylation in lysosomal enzyme routing has also been studied. An acidic acetyltransferase which can acetylate MSH has also been detected in secretory vesicles.